The invention concerns a reflection-photometric analytical system having a measuring head comprising a source of radiation and a radiation detector for the reflectometric analysis of a target surface of a test object arranged at a distance from the measuring head and in particular of a test strip for body fluids such as urine or blood. The invention also concerns a corresponding measuring method.
Systems of this type are used particularly in the field of clinical analysis in order to determine the presence and concentration of certain analytes in a sample liquid by means of test strips. A spectroscopic analysis of radiation reflected from irradiated test fields of the test strips is carried out in automated instruments. A critical factor for reflectometric measurement is a constant measuring distance where it is necessary to take into account that the test fields can have different heights and that absorption of the sample liquid can result in different degrees of swelling. It has been attempted to reduce the distance dependency by using a receiving optical system having a long focal length and a greatly reduced optical aperture which is aligned perpendicularly to the target surface. However, this requires a large space and the signal currents of the photodetector are small and thus require complicated measurement instrumentation to process them.
A sensor for distance measurement and a control unit for adjusting the measuring distance has already been proposed in JP-A 03-166738 (publication number 10948/1993) in a similar context. However, it does not disclose anything about the operating principle of the distance detection. A general problem is that the surface properties of test strips can vary greatly due to the application of sample liquid since almost dry, rough surfaces as well as wet and hence shiny surface may be present.
This was the starting point for the invention whose object is to avoid the disadvantages of the prior art and to improve an analytical system and a method of the type described above in such a manner that a high measuring accuracy is achieved even when the target surface has a variable height profile. In particular it should enable a reliable control of the measuring distance.
The combination of features stated in the claims are proposed to solve this problem. Advantageous embodiments and further developments of the invention are derived from the dependent claims.